The present invention relates to a bearing assembly for an axle shaft of a vehicle having a sensor for detecting the speed of rotation of a wheel attached to the axle shaft so as to control the operation of a brake ring in accordance with the speed of the wheel so that the wheel will not lock when it is suddenly braked, for example on a frozen road surface.
A prior art bearing apparatus of this kind is shown in FIG. 3. The bearing apparatus comprises an inner ring 51 consisting of two adjacent portions and fixed to an end portion of an axle shaft 50 of a freely rotatable wheel 54; an outer ring 55 connected to the wheel 54 at one end thereof; and a plurality of rolling elements or bearings 53 in two rows, the rolling elements in each row being positioned between the inner ring 51 and outer ring 55 and spaced a distance from each other by a retainer 52 in the circumferential direction of the inner and outer rings 51 and 55. The bearing apparatus is also provided with a revolution detection ring 56 which is fixed to the outer surface of the outer ring 55 at the other end thereof and which cooperates with a sensor 57, described later, for detecting the number of revolutions of the wheel. The revolution detection ring 56 is made of a ferromagnetic material and has projections like the teeth of a gear on the outer surface thereof. The sensor 57, which houses a permanent magnet and a coil, is fixed to the base of the axle shaft 50 at a slight distance from the outer surface of the revolution detection ring 56. The sensor 57 detects the number of revolutions of the wheel 54 by cooperating with the revolution detection ring 56. That is, when the revolution detection ring 56 rotates along with the wheel 54, magnetic flux generated from the permanent magnet of the sensor 57 is varied by the gear-shaped projections formed on the outer periphery of the revolution detection ring 56. As a result, an alternating voltage of a frequency proportional to the revolution speed of the revolution detection ring 56 is generated in the coil of the sensor 57. Therefore, the revolution speed of the wheel 54 is determined by detecting the frequency of the alternating voltage.
However, it is necessary to adjust the gap between the revolution detection ring 56 and the sensor 57 finely when the bearing apparatus is mounted on the axle shaft 50, because the revolution detection ring 56 is mounted on the outer ring 55, whereas the sensor 57 is mounted on the base of the axle shaft 50 to be spaced from the outer peripheral surface of the outer ring 55. Therefore, the production rate of such a bearing apparatus for an axle shaft is not high. Further, the space occupied by the bearing apparatus is large because the revolution detection ring 56 is mounted on the outer peripheral surface of the outer ring 55 and the sensor 57 is required to be near the revolution detection ring 56. In addition, since the sensor 57 secured to the base of the axle shaft 50 projects from a protection cover 58 of the vehicle, when earth, sand, muddy water, or the like enters the gap between the protection cover 58 and a brake shoe 59 fixed to the outer ring 55, and advances as shown by the arrow A, a tip portion of the sensor 57, defining a pulse detecting portion, and the gear formed on the outer peripheral surface of the revolution detection ring 56 are damaged, which causes an inaccurate detection of the revolution speed of the wheel 54.